Ribbon bonding

ABSTRACT

A flexible conductive ribbon is ultrasonically bonded to the surface of a die and terminals from a lead frame of a package. Multiple ribbons and/or multiple bonded areas provide various benefits, such as high current capability, reduced spreading resistance, reliable bonds due to large contact areas, lower cost and higher throughput due to less areas to bond and test.

BACKGROUND

[0001] 1. Field of the Invention

[0002] The present invention relates to semiconductor devices, and inparticular, to interconnecting a semiconductor die to a terminal lead ina semiconductor package.

[0003] 2. Related Art

[0004] In the manufacture of semiconductor devices, active elements in asemiconductor device, such as drain and/or source regions in asemiconductor die, are electrically connected to other devices orelectronic components, such as on a printed circuit board. However,since semiconductor devices can be susceptible to environmentalconditions, such as dust, moisture, and sudden impact, which can damageor otherwise interfere with the proper operation of the device, thedevice is typically protected by a die package. The die package bothprotects the die and allows the die to electrically connect to externaldevices. To facilitate the latter, specific portions of the die areelectrically coupled to external leads of the package or lead frame,such as with bond wires or solder balls.

[0005]FIG. 1 shows a side view of a typical connection between a powersemiconductor die 10 (e.g., a MOSFET) and part of a lead frame 12. Leadframe 12 includes a lead 14 and a die pad 16. Lead 14 allows die 10 toelectrically couple external elements after die 10 is connected. Die 10is mounted on or secured to die pad 16. The upper surface of die 10includes a metalized portion 22, such as aluminum, that provides contactwith underlying active elements of die 10. An electrical connection isthen made between metalized portion 22 and contact portion 18.Typically, the connection is made by bonding, e.g. ultrasonicallybonding, a conductive wire 24 between the two portions. Materials forwire 24 include gold, aluminum, and copper. FIG. 1 shows a single bond,connection, or stitch 26 between wire 24 and metalized portion 22.

[0006] The amount of current flow from die 10 to lead 14 depends, inpart, on the total resistance in the current path, as shown by thearrows in FIG. 1. This resistance is due, in part, to the resistance ofwire 24 and the spreading resistance along metalized portion 22. Thespreading resistance increases as the distance the current has to travelfrom the metalized portion to the stitch increases. The spreadingresistance also increases as the thickness of metalized portion 22decreases. Typical metalization thickness is in the range ofapproximately 3 to 5 microns (i.e., much smaller than the wirethickness). It is desirable to lower the overall electrical resistanceof the connections, especially to keep pace with the intrinsicresistance of the semiconductor die, which is continuously decreasing.However, increasing the thickness of metalized portion 22 also increasesthe cost by decreasing throughput of the wafer/die manufacturingprocess.

[0007] Further, wires are limited by their size, typically around 20 milin diameter, which also limits the amount of current that can be carriedin each wire. Consequently, large numbers of wires are sometimes neededto make the desired connections in certain applications, which canincrease the cost and decrease throughput of the interconnect processequipment (e.g., the wire bonder).

[0008] Instead of wires, other types of bonding utilize a strap toconnect the die to the lead frame. One such configuration is shown inFIG. 2 and is disclosed in U.S. Pat. No. 6,040,626, entitled“Semiconductor Package”, issued to Cheah et al. A single conductivestrap 50, e.g., copper, is used to obtain an electrical connectionbetween metalized portion 22 on die 10 and lead 14 of thepackage/module. Strap 50 can be either soldered or glued to two contactareas 52 and 54, such as with an electrically conductive epoxy or solderpaste 56. Use of a strap provides the advantages of reducing resistanceto current flow by providing a large contact area for coupling metalizedportion 22 to lead 14, e.g., spreading resistance is greatly reduced.

[0009] However, using a strap also has disadvantages. In order to solderstrap 50 to the surface of metalized portion 22, a solderablemetalization, e.g., copper or nickel, is required. In general, such ametalization requires a stack of several different metal layers (notshown), with each layer having a specific function, e.g., adhesion,barrier, and solderability, of the soldering process. These layers,which are different than the standard metalization layer, e.g.,aluminum, together result in higher manufacturing cost of themetalization, and consequently of the semiconductor die. Typically, asolder paste process is applied to join the parts. Solder paste 56contains some type of flux component, which is required to (1)temporarily tack the components, (2) protect them from oxidizing(especially if the reflow process takes place in air), and (3)remove/reduce oxides already present. Depending on the quality of theparts, only the use of a strong flux provides a robust process andreliable result of the soldering process. It is well known that fluxresidues cover the surfaces after reflow. Beside other negative effects(like corrosion in contact with humidity), their presence negativelyinfluences the strength and reproducibility of the adhesion of themolding compound in a subsequent package encapsulation. This again canresult in a limited reliability of such parts. As a consequence, partsprocessed with solder paste typically need to be thoroughly cleanedafter reflow and before further processing/packaging.

[0010] However, cost effective wet chemical cleaning processes are knownto offer limited process control capability, causing an increased yieldloss potential, beside the additional costs (e.g., labor, floor space,equipment, consumables, and yield loss) due to the need for thisadditional process step. Such a cleaning is also difficult to automate(which would reduce labor cost) and difficult to implement in a cleanroom environment. Furthermore, wet chemical processes, as well as solderreflow using flux (fumes), may be environmentally unfriendly. Two otherdisadvantages of a copper strap interconnect are (1) limited flexibility(since the straps are typically stamped on the die bonder, devicechanges which require a different strap geometry will require exchangingthe stamping tool, which increases time and cost) and (2) the relativelystiff copper strap can form a significant stress on the silicon die,which can cause the die to crack, especially if the thickness of theattachment layer (e.g., solder or epoxy) is not well controlled above acertain minimum.

[0011] Another type of interconnect currently used is a solder ballbased interconnect, such as disclosed in U.S. Pat. No. 6,442,033,entitled “Low-cost 3D Flip-chip packaging technology for integratedpower electronics modules”, issued to Liu Xingsheng et al., and in U.S.Pat. application publication No. US 2002/0066950, entitled “Flip chip inleaded molded package with two dice”, by Rajeev Joshi, both of which areincorporated by reference in their entirety. Solder ball basedinterconnects have similar disadvantages to those of the strapconfiguration with regards to the use of solder paste and inflexibility.In high current applications, such a configuration has the additionaldisadvantage of the solder's high susceptibility to electromigration.

[0012] Thus, it is highly desirable to use a clean, environmentallyfriendly process, which can be well controlled, as well as a flexibleinterconnect. The ultrasonic bonding process is one such process.However, it is also desirable to reduce the number of connections, inorder to increase the production rate of existing equipment, and reducethe cost of manufacturing. Furthermore, it is desirable to improve theelectrical performance of connections, which would require eitherreducing the electrical resistance and/or increasing the currentcapability, depending on the type of application. Especially fordiscrete semiconductor devices, it is also desirable to reduce theoverall size of a device, and therefore the volume required by theconnection.

[0013] Accordingly, there is a need for an improved type of connectionprocessed using ultrasonic bonding, which overcomes the deficiencies inthe prior art as discussed above.

SUMMARY

[0014] According to one aspect of the present invention, a flexibleconductive ribbon is used to electrically connect a die and an externallead in a package, such as for power applications and MOSFETs. Theconnection to the die is by ultrasonic bonding in one embodiment.Bonding of the lead may also be by ultrasonic bonding. Other embodimentsmay utilize thermosonic bonding. The ribbon is of a rectangularcross-section and can be of a single layer, such as aluminum or copper,or can be of multiple layers, such as a thin aluminum bonding layerunderlying a thicker copper conducting layer. In some embodiments, asingle ribbon is used, while in other embodiments, multiple parallelribbons are bonded. Further, each ribbon can have one or more stitchesor bonds on the conductive die surface with one or more loops.

[0015] The present invention allows using the same bonding process aswith wires, i.e., an ultrasonic bonding process, increasing the crosssection and contact area for the current paths while limiting or evenreducing the overall volume occupied by the connection, and reducing theprocessing steps and time to produce the connection. Using a ribbon ofthe present invention allows the main loop cross section to be maximizedsince gaps between wires are filled. Even though the ribbon has a largecross section, the thickness can still be reduced compared to a wire,which makes bonding by ultrasonic processes less difficult.Consequently, bonding is easier, the loop height can be lowered, whichresults in lower interconnect height and a potential for reduced packageheight for smaller packages, and the ability to form the loopsincreases, which results in shorter loops and more stitches. Multiplestitches allow using smaller bond areas (per stitch), which alleviatesthe need for a heavy bond head to generate and apply high force andultrasonic power during bonding. Thus, yields higher throughput. The useof multiple stitches, leading to shorter distances between stitches,reduces the spreading resistance, which allows higher current carryingcapability.

[0016] In other embodiments, the ribbon can be bonded and cut atdifferent angles, depending on the orientation of the contacts of thedie and lead terminals. This allows optimal usage of the ribbon contactarea, maximizes ribbon width, and allows placing a large ribbon inexisting packages designed for round wire bonding (e.g., TO-220packages) in an optimized manner.

[0017] This invention will be more fully understood in conjunction withthe following detailed description taken together with the followingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 shows a side view of a conventional connection between adie and a lead frame using a wire with a single stitch;

[0019]FIG. 2 shows a conventional connection between a die and a leadframe using a strap;

[0020]FIGS. 3A and 3B show side and top views, respectively, of aconnection between a die and a lead frame according to one embodiment ofthe invention;

[0021]FIG. 4A is a side view of a connection between a die and a leadframe using a ribbon with multiple stitches according to anotherembodiment;

[0022]FIG. 4B is a side view of a ribbon with multiple stitches, showingspacing between stitches according to one embodiment;

[0023]FIGS. 5A-5C show top views of another embodiment of the presentinvention using multiple parallel ribbons;

[0024]FIG. 6 shows a cross-sectional view of a clad ribbon according toone embodiment of the invention;

[0025]FIG. 7 shows a cross-sectional view of a clad ribbon with threelayers according to another embodiment of the invention;

[0026]FIG. 8 shows a portion of a bond tool suitable for ribbon bondingaccording to one embodiment;

[0027]FIG. 9A shows a top view of a connection between a die and a leadframe where the contacts are aligned;

[0028]FIG. 9B shows a top of a connection between a die and a lead framewhere the contacts are not aligned using a conventional process;

[0029]FIG. 9C shows a top view of the contact alignment of FIG. 9B usingan embodiment of the present invention;

[0030]FIG. 10 shows a top view of a device with gate fingers where aribbon has stitches between the gate fingers according to oneembodiment; and

[0031]FIG. 11 shows a side view of stacked ribbons according to anembodiment of the present invention.

[0032] Use of the same or similar reference numbers in different figuresindicates same or like elements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] According to one aspect of the present invention, one or moreconductive flexible ribbons are used to electrically connect asemiconductor die to a lead frame.

[0034]FIGS. 3A and 3B show a side view and top view of a semiconductordie 400 coupled to a lead frame 402 by a conductive flexible ribbon 404,according to one embodiment of the invention. Die 400 includes ametalized portion 406, such as aluminum, that provides connection tounderlying elements of die 400. Die 400, in one embodiment, is part of apower semiconductor device, such as a power MOSFET. Lead frame 402includes a support portion 407 to which die 400 is secured, e.g., bysolder or epoxy, and terminals 408 that allow electrical connection toexternal devices. An enclosure, covering, or package protects the diefrom external elements. In some embodiments, the semiconductor packageis a TO-220 or an SO-8 package.

[0035] Ribbon 404, which may have a rectangular cross section, isaluminum, although other conductive metals, such as copper, are alsosuitable. In one embodiment, the mechanical properties of ribbon 404 aresimilar to that of wire. For example, a 60 mil×8 mil aluminum ribbon mayhave a tensile strength of approximately 2000 g. The width of ribbon 404may range from 20 mil to 100 mil or more. In one embodiment, the widthis 120 mil. Larger width ribbons generally are able to replace largernumbers of wires. For example, a single 120 mil ribbon may replace five20 mil wires. The thickness of ribbon 404 may range from 2 mil to 10 milor more. In one embodiment, the thickness is 12 mil. Thicknesses of 2mil require precise cutting control so that the ribbon can be cut whilenot cutting into the substrate. Note that lower thicknesses may bepossible with improved equipment and processes. Some typical sizes ofribbon 404 are 20 mil×2 mil, 20 mil×4 mil, 30 mil×3 mil, 40 mil×4 mil,50 mil×5 mil, 60 mil×8 mil, 80 mil×6 mil, 80 mil×8 mil, 80 mil×10 mil,and 100 mil×10 mil. Aspect ratio (width/thickness), in one embodiment,is between 7 and 13, with a typical ratio of approximately 10. An aspectratio of approximately 10 has been shown to provide a good compromisebetween bondability (the thinner the better) and tilt sensitivity (thethicker the better). Those skilled in the art will appreciate that othersizes of ribbon may also be suitable, depending on factors such assystem requirements and process technology.

[0036] Ribbon 404 is bonded to metalized portion 406 by an ultrasonicbonding process, as is known by those skilled in the art, and isdisclosed, e.g., in commonly-assigned U.S. Pat. No. 4,824,005, entitled“Dual mode ultrasonic generator in a wire bonding apparatus” to Smith,Jr. and U.S. Pat. No. 6,439,448, entitled “Large Wire Bonder Head” toRingler, both of which are incorporated by reference in their entirety.Ultrasonic bonding is more environmentally friendly, cleaner, lessexpensive, and easier to control than soldering. However, ultrasonicbonding becomes more difficult as the thickness of the wire or ribbonincreases. In the case of a wire, small diameter wires can be used, buta larger number of wires are needed to obtain the same cross section,thereby resulting in lower productivity and higher cost.

[0037] The relatively small thickness of ribbon 404 (e.g., 2 to 10 mil)allows ultrasonic bonding, while a large width (e.g., 20 to 100 mil)allows a large bonding area. The small thickness of ribbon 404 alsoprovides flexibility so that, within a given area of metalized portion406, the number of bonds or stitches can be increased and the length ofribbon between bonds can be decreased. This can be advantageous, as willbe discussed in more detail below.

[0038] Referring to FIG. 3B, ribbon 404 is ultrasonically bonded, in oneembodiment, to metalized portion 406 along an area 412, although theactual bond may be smaller. The ultrasonic bonding is by conventionalprocesses, e.g., with frequencies around 60 kHz. However, variations ofcurrent ultrasonic bonding processes are also suitable. For example,ultrasonic bonding can be at a frequency higher than 60 kHz, e.g.,around 80 kHz, which provides higher reproducibility of the resultingbond and requires a smaller vibration amplitude from the bond tool. Inother embodiments, ribbon 404 is thermosonically bonded to metalizationportion 406. There are trade-offs in using thermosonic bonding, such asadditional complexities and costs associated with heating a substrateand bonding on the heated substrate versus lower ultrasonic powerrequirements and a more forgiving process.

[0039] Referring back to FIG. 3B, area 412 is approximately the width ofribbon 404 and approximately twice the thickness of ribbon 404 in oneembodiment. In other embodiments, the length of area 412 is less thantwice the thickness of ribbon 404, but more than the thickness of ribbon404. In some embodiments, the actual bond from the ultrasonic bonding isat least as large as the area of the cross-sectional area of ribbon 404.A bond tool for a larger (longer) bonded area 412 allows achieving abond of sufficient length (i.e., at least as long as the thickness ofthe ribbon) even with less than optimal ultrasonic bonding parameters.Further, the actual bond area is of the same magnitude as the bond areafor wire bonding, and thus, comparable force (bond) and power capability(ultrasonic) to wire bonding can be used for ultrasonic bonding ribbon404. Typically, sufficient current flow is obtained when the actual bondarea is at least approximately the size of the ribbon's cross section.

[0040] In general, performance and processability increase as the widthof ribbon 404 increases. However, the width is limited by variousfactors, such as the ultrasonic bonding process, reliability of thebond, and the type of package. For example, the width of the ribbon andits aspect ratio must be limited depending on the tilt of the bondsurface relative to the bond tool. Stress and strain caused by a thermalexpansion mismatch between the ribbon and the substrate increase withincreasing size of the bonded area, progressively limiting the fatiguefailure lifetime of a bond with increasing maximum linear dimension. Thecritical linear dimension is the length of the diagonal of therectangular bond area. The severity of this limitation depends on thetype of package and the type of application. It will be less severe in aplastic molded discrete package, but more pronounced in an electronicmodule (which is typically filled with silicon gel only to achieveprotection against oxidation and corrosion). However, as long as themaximum dimension is comparable, e.g., within ±50%, with the maximumdimension of the bond area of a large wire, this limitation is expectedto be comparable to the one for a round wire bond because the largestdimension is still of similar size.

[0041]FIG. 4A is a side view showing another embodiment of the presentinvention in which multiple bonds or stitches 500 are used to contactribbon 404 to metalized portion 406. Stitches 500 are formed usingultrasonic bonding in one embodiment. Multiple stitches reduce spreadingresistance along metalized portion 406. As seen from FIG. 4, the shorterthe distance between stitches 500, the less distance current has totravel in the metalization with high electrical resistance, resulting inless spreading resistance and higher current flow to terminals 408 oflead frame 402. In one embodiment, the number of stitches is between 2and 6, although a single stitch may also be used. The distance betweenstitches 500 depends on various factors, such as the size of ribbon 404,the ribbon bonding process, and requirements of the resulting device.For example, thinner ribbons allow the distances to be shorter, due inpart to increased flexibility. In one embodiment, the distance betweenstitches (center-to-center of the stitches) for an 80 mil×8 mil ribbonis between approximately 1.25 mm and 2 mm, and for a 40 mil×4 milribbon, the distance may be reduced down to 0.6 mm.

[0042]FIG. 4B is a side view of an embodiment with multiple stitches forillustrating stitch placement on the die. Ribbon 404 is a single wideribbon (approximately the width of metalized portion 406) or one ofmultiple parallel ribbons. Distances x₀, x₁, x₂, x₃, and x₄ are asshown, where x₀+x₁+x₂+x₃+x₄=d (the length of metalized portion 406).Note that only four stitches 500-1 to 500-4 are shown, although othernumbers are also possible. Further, the distances x₀ to x₄ are not shownto scale, as different embodiments will yield different distances. Inone embodiment, the distances x₀ and x₄ (from the center of the stitchesat the ends of the die to the edge of the die) are approximately halfthe distance of the center-to-center distance of the interior stitches,i.e., x₁=x₂=x₃=2x₀=2x₄. Placement in this manner, to a firstapproximation, minimizes the maximum distance between any point onmetalized portion 406 and the nearest bond or stitch 500. Thisapproximation also assumes the resistance of each of the small loops(between stitches 500) is much less than the resistance of the main loop(between stitch 500-1 and terminal 408). The accuracy of theapproximation increases as the number of stitches increases, i.e., adenser placement of stitches.

[0043] Among the stitches, resistance is lowest at stitch 500-1,resulting in the highest current flow and highest current density atstitch 500-1. Thus, in applications that are limited by the peakcurrent, damage may appear at the first stitch, e.g., in the form ofmelting of die metalization and damage of the die due to too high acurrent density at the bond or stitch. Consequently, it would bedesirable to place stitches 500 such that each stitch “sees” the sameresistance or current flow. Thus, according to another embodiment,stitches 500 are placed such that the separation between stitchesincreases as the stitches move farther away from terminal 408. In oneembodiment, x₀<x₁/2<x₂/2<x₃/2<x₄.

[0044] The distance may also be limited by the process in which ribbon404 is bonded to metalized portion 406. For example, during the bondingprocess, ribbon 404 is fed through a bond tool, as will be described inmore detail below, to the bond area, where ultrasonic bonding securesribbon 404 to metalized portion 406. Additional ribbon 404 is then fedthrough the bond tool to form a loop and down to the next bond area.After the first bond, if the ribbon is looped back from the direction ofthe feed, a shorter distance to the next bond or stitch is possible.However, by looping the ribbon back, stress is placed on the new bond,which may damage or break the bond. Looping the ribbon forward in thesame direction as the feed greatly reduces stress, but also makes theformation of a short loop more critical. In one embodiment, the ribbonis directed at an approximately 90° angle from metalized portion 406,which places a limited level of undue stress on the bond while alsoallowing a short distance between bonds. As will be appreciated by thoseskilled in the art, however, feeding the ribbon forward or backward atvarious angles may be the most desirable depending on the variousfactors such as the bonding equipment, the ribbon, and the devicerequirements.

[0045] There are also advantages to minimizing the height H of theribbon loops. A lower height allows a smaller profile package, as wellas reduced resistance for higher current flow. However, as with thedistance limitation, the height is limited by the thickness of theribbon as well as the bonding process. There is no upper limit of theloop height (within the range of typical dimensions of semiconductorpackages or modules), but the lower the target loop height, the morechallenging its control, i.e., to achieve a high reproducibility, theminimum loop height will depend on the thickness of the ribbon (via theinfluence on the stiffness with regards to bending the ribbon). In oneembodiment, loop heights are 1.00 mm (from surface of the die to topsurface of the ribbon) for 8 mil thick ribbons. However, depending ondevice requirements, loop heights can have other heights, such as 0.60mm to achieve reproducibility or sufficient control of the loop heightor 0.40 mm for an 8 mil thick ribbon to allow filling material (e.g.,silicon gel in power modules and plastic mold in discrete power devices)still enough space to properly fill so that voids/bubbles do not formunder the ribbon.

[0046] Factors other than device and/or process limitations may alsodetermine the number of and distance between stitches on metalizedportion 406. Even though shorter distances between stitches reduce theresistance and provide higher current, the larger number of stitches orloops also decreases throughput. For applications in which a highcurrent is not critical, a higher throughput may be more desirable atthe cost of lower current flow. In such a situation, a lower number ofstitches or bonds would be formed. Therefore, the number of stitches anddistance between stitches may vary depending on the device requirements.

[0047]FIGS. 5A, 5B, and 5C show top views of another embodiment of thepresent invention, in which multiple ribbons 404 are used to connectmetalized portion 406 to terminals 408. FIG. 5A shows four 60 mil×8 milribbons 404 with three stitches or bonds, FIG. 5B shows four 80 mil×8mil ribbons with four stitches, and FIG. 5C shows four 80 mil×8 milribbons with five stitches. Higher current carrying capability or lowerresistance is possible with multiple ribbons since the area from whichthe current to the bond is collected is reduced. In general, as thenumber of ribbons increases, the resistance decreases and current flowincreases. However, as the number increases, the required width of theindividual ribbons can decrease. Narrower ribbons are desirable when thedevice is subject to large temperature variations. This is because asmaller bond experiences less stress from coefficient of thermalexpansion (CTE) effects, resulting in a more reliable bond. Thus,tradeoffs exist between having fewer wider ribbons versus having alarger number of narrower ribbons. As discussed above, a typical aspectratio is approximately 10. Lower aspect ratios may also provideadvantages over wires; however, as aspect ratios decrease (e.g., to 3 orless), the ribbon becomes to look and behave like a wire, therebynegating the advantages provided by ribbons.

[0048] Typically, as for a round wire, there is a CTE mismatch betweenthe ribbon and the underlying silicon. For example, a copper ribbon maybe desirable because copper has a lower resistance than aluminum and hasa higher melting point. However, when the metalization is aluminum,which is softer than copper, bonding the copper ribbon to the aluminumlayer may result in the bond extending through the aluminum layer anddamaging the underlying silicon (this would be even worse for a roundwire, due to the higher pressure). Therefore, in one embodiment, acopper plating or a metal plate is placed over the metalization layer.The metal plate should be a material having a CTE between that ofsilicon and the ribbon material, e.g., copper, to act as a stress/strainbuffer. In one embodiment, the material is nickel-plated molybdenum.This eliminates the need to reduce spreading resistance, which resultsin the resistance mainly residing in the ribbon. A wide copper ribbonwith one stitch provides a large bonding contact area for a reliablebond, while providing low resistance for current flow. Further, using amaterial having a closer CTE to the bond surface (copper) reduces thetemperature effects on the strength of the bond.

[0049]FIG. 6 shows a cross-sectional view of one embodiment of a cladribbon 600 formed from a thin first conductive layer 602 and anoverlying second conductive layer 606. First layer 602 is made of thesame material as the underlying metalization layer 604 to which it is tobe bonded. Properties of first layer 602 or bonding layer includeincreased bondability (“soft” so that underlying structures remainundamaged and easily bondable with ultrasonic processes), corrosionresistance, high electrical conductivity so that current can easy travelthrough this layer to reach second layer 606, and relatively lowcoefficient of thermal expansion (or near to silicon). In general, thelatter two properties are somewhat less important than the first two. Inone embodiment, aluminum is used for first layer 602 having a thicknessof approximately 2 mil. Thus, two like-materials are bonded together(when the metalization is aluminum), which results in a stronger andmore reliable bond.

[0050] Furthermore, since it is currently not possible to directly bondcopper to aluminum metalization layers which overlie active circuitry,aluminum bond pads are typically moved to areas where there is no activecircuitry underneath. This can allow bonding of thin (e.g., 2 mil)copper wires to aluminum metalization without the danger of damagingactive elements underneath the aluminum. However, moving bond padsnormally requires making the silicon die larger. Size of the silicon dieis still the major cost factor in a semiconductor device. Therefore, thecapability to bond copper over active circuitry with a high yield isvery desirable, which can be accomplished using a clad ribbon with analuminum layer between the metalization layer and the copper ribbon.

[0051] A thicker second layer 606 having higher thermal and electricalconductivities and lower resistance overlies first layer 602. Besideshigh conductivity, second layer 606 should also be corrosion resistantand have a low coefficient of thermal expansion. An additional property,sometimes not as important as the above, is to limit its hardness inorder to not influence loop forming capability too much and to allow agood coupling between the layer and the bond tool. In one embodiment,second layer 606 is copper having a thickness of approximately 6 to 8mil. Second layer 606 can have different thicknesses, depending, in someembodiments, on the thickness of first layer 602 such that the aspectratio is within 7 and 13 (typically 10). Larger cross sections (greaterthickness) will require higher ultrasonic power for bonding andtherefore higher force to reach the necessary coupling between thebonding tool and the copper portion. Copper provides low electricalresistance and/or a strong/stiff and corrosion resistant loop. Othermaterials that may be suitable for second layer 606 include gold, whichis much more costly, and a silver-nickel alloy.

[0052] A 2 mil aluminum bonding layer has been found to be suitable withcurrent processes. Aluminum is desirable for ultrasonic bonding becauseit can be joined with many materials at room temperature, is easilybondable, and protects underlying active elements from possible damagefrom ultrasonic bonding. Its “softness” enables bonding to sensitivestructures with high yield. While its electrical and thermalconductivities are high, they are still lower compared to some othermaterials like copper. However, while copper has higher electrical andthermal conductivities, it is relatively hard and difficult to bond.Thus, forming ribbon 600 with a thin aluminum layer between the copperand the aluminum metalization provides advantages of both the copper andthe aluminum. In other embodiments, first layer 602 can be of a metal orbond material similar to but not exactly the same as the underlyingmetalization, which will still yield benefits, although not to theextent of using the same metal. In another embodiment, copper is used.Other embodiments may utilize a harder bonding layer than aluminum, suchas in the case when the metalization underlying the active elements isformed of a harder material, such as copper.

[0053] Another design aspect of power interconnects is theirreliability. Especially in power modules, the thermal mismatch at thebond interface, mainly caused by the large CTE difference betweensilicon and aluminum is a limiting factor. Theoretically, this could bedrastically changed with the clad ribbon 600 discussed above, if themain ribbon material or thicker second layer 606 has a CTE nearer to theone of silicon. For example, since the CTE of copper (i.e., ˜17×10⁻⁶K⁻¹) is nearer to the one of silicon (i.e., ˜3×10⁻⁶ K⁻¹) than aluminum(i.e., ˜24×10⁻⁶ K⁻¹) so that the difference is ˜14×10⁻⁶ K⁻¹ compared to˜21×10⁻⁶ K⁻¹, the reliability should be improved. Calculations haveshown an approximate factor of two improvement. As an example, thisoffers the potential to improve the reliability in industrial powermodules by approximately a factor two, a long-sought after improvement.

[0054]FIG. 7 shows another embodiment of a clad ribbon 700, in which athird layer of material 702 overlies second layer 606. As with theembodiment of FIG. 6, first layer 602 is of a bond material, e.g.,conducive to ultrasonic bonding, and second layer 606 is of a conductivematerial, chosen for various aspects such as conductivity andflexibility. As the coupling between the bond tool and ribbon 700 isanother key requirement (which determines the required force to asignificant extent), it may be beneficial to include third layer (orbonding layer) 702, which is not necessarily identical to first layer602 at the bottom. Third layer 702 would support an optimized couplingbetween the ribbon and the bond tool. Properties of third layer 702 orcoupling layer are selected to have medium hardness, be corrosionresistant, have high electrical conductivity, and a low coefficient ofthermal expansion, although typically the first two properties havegreater importance than the latter two. However, this may change withdifferent applications. In one embodiment, aluminum is used for thirdlayer 702. Other materials may include copper, gold, and silver,although each has disadvantages. For example, copper is hard and goldand silver are more costly. The thickness of third layer 702, in oneembodiment is between approximately 0.5 mil and 2 mil, with a typicalthickness being 1 mil. First layer 602 and second layer 606 are similarto that described above with respect to the embodiment of FIG. 6.

[0055] One advantage of using a ribbon instead of a round wire in a cladconfiguration is that aluminum is more effectively utilized. Forexample, using a round wire with a copper core and an aluminum cylindersurrounding the copper, only the bottom portion of the aluminum is usedfor bonding when the wire is bonded and “flattened” against themetalization. Thus, the upper and side portions of the aluminum are notused to create the bond. However, using a ribbon, the aluminum is fullyutilized during the ultrasonic bonding process. Accordingly, more copper(as a percentage of the total wire cross-section) can be used, resultingin a higher current carrying capability. The thickness of the clad layerof a ribbon can be chosen lower than for a wire (e.g., for a two layeror one-sided clad ribbon), because it has to deform much less (less than1 mil, typically approximately 0.5 mil, according toobservations/investigations).

[0056]FIG. 8 shows one embodiment of a portion (the foot) of a bond toolfor use in the present invention. The foot of the bond tool utilizes across groove structure and forms a diamond pattern, as shown. The depthof the grooves depends on the thickness of the ribbon. In oneembodiment, the depth is between approximately 1.0 mil and 1.5 mil foran 8 mil thick ribbon. This pattern type increases the coupling surfacebetween the tool and the ribbon during the bond process. The edges ofthe grooves also improve the immediate locking between the tool and theribbon at the start of the bond process, reducing the slippage betweenthe tool and the ribbon, and transferring the tool's motion into theribbon and to the interface between the ribbon and the substrate. It isalso expected that this diamond pattern slightly reduces the tiltsensitivity of the ribbon bonding. The bond surface should be wellperpendicular to the bond tool; otherwise the load of the tool onto theribbon is inhomogeneous, and bonding, then actually over-bonding, takesplace only on one side of the ribbon. The diamond pattern limits thedeformation on the side where the tool is nearer to the substrate (dueto a limited perpendicularity), but because the tool still easier sinksinto the material on that side of the ribbon, it will more likely alsosink in to some extent on the other side, and transfer at least someenergy. Thus, this type of pattern increases/maximizes contact areabetween the tool and the ribbon and minimizes damage to the ribbon heel.It has also been observed that using ribbons with aspect ratios of 10provides an acceptable bond quality for tilt angles up to 1°.

[0057] In many packages, terminals 408 and metalized portion 406 are“aligned”, as shown in FIG. 9A, along a line 900. Aligned, as usedherein, means a majority of the bonding area of terminals 408 are withinthe area projected from the bonding area of metalized portion 406perpendicular to one of its sides. In these types of packages (e.g.,SO-8 packages), ribbon 404 is fed and bonded along line 900 and cutperpendicular to line 900. Here, the maximum width of ribbon 404 islimited by the lesser of the two widths of metalized portion 406 andterminal 408. However, other types of packages may have terminals 408and metalized portion 406 that are offset from each other, such as shownin FIG. 9B. Using conventional ultrasonic bonding equipment andprocesses, ribbon 404 is bonded and cut perpendicular to the length ofthe ribbon, as shown in FIG. 9B. This may reduce the bonding area andwidth of the ribbon, as well as the number of stitches or bonds, sincethe area on terminal 408 and metalized portion 406 is not optimallyutilized.

[0058]FIG. 9C shows one embodiment of the invention in which ribbon 404is cut and bonded parallel to the intended bond area of terminal 408 andmetalized portion 406. Using this configuration, bonding areas are morefully utilized so that wider ribbons can be used with more bonds.Conventional ultrasonic bonding equipment can be modified so that thebond tool is rotated. This allows the ribbon to be bonded and cut atdifferent angles. By cutting and bonding in parallel with the bondingareas of the die and terminals, the ribbon width, bonding area, and/ornumber of stitches can be advantageously increased. Note thatorientations other than parallel may also provide benefits over bondingwith a fixed tool.

[0059] Angle bonding can be accomplished by making modifications toexisting ultrasonic bonding equipment, such as described incommonly-assigned U.S. Pat. No. 4,976,392, entitled “Ultrasonic wirebonder wire formation and cutter system”, which is incorporated byreference in its entirety. Angle bonding can be achieved by rotating theribbon guide and the cutter relative to the bond tool or by rotating thebond tool alone (however then the cut is not parallel to the bond(tool)).

[0060] One way to achieve an angle bond is by rotating the bond foot ofthe tool relative to the transducer and wire guide. Although this waythe orientation of the tool and therefore the setup is fixed andapplication specific, it does not mean any other additional effort. Ofcourse the vibration characteristics will be different and a function ofthe angle, but this can be accommodated for. Also the cutter is rotatedif a cut parallel to the bond is required. Such a setup is sufficient inmost discrete power applications, where typically only one angle isrequired (see FIG. 9C). In a power module application, where there maybe many more interconnects under different angles, a flexible solutionmay be required. This would allow adjusting the relative angle betweenbond foot and ribbon guide and cutter, thereby supporting theflexibility, which is the major strength of ultrasonic bonding comparedto other interconnect techniques in such applications.

[0061]FIG. 10 shows another embodiment of the present invention, inwhich ribbon 404 with multiple stitches 500 (of FIG. 5) is used with adevice, such as a MOSFET die, with gate fingers 1000. Stitches 500 arebonded between gate fingers 1000. This eliminates the need to form anadditional insulating layer over fingers 1000. One trend, also in powerelectronics (mainly discrete power MOSFET for DC-DC converterapplications), is to higher switching frequencies. Higher frequencyoperation (e.g., >1 MHz) improves the efficiency in DC-DC converterapplications and allows keeping passive components (e.g., inductors andcapacitors) smaller.

[0062] To improve the switching behavior of a MOSFET, gate fingers aredesigned into the area of the source metalization (in order to reducethe distance between the gate and any point in the source to reduce theswitching delay). These gate fingers interrupt the source metalization.For example, U.S. Pat. No. 6,040,626 describes how these fingers need tobe covered with some electrically isolating material such that the clipattached to the source metalization does not create a short between thegate (finger) and the source metalization. If the gate fingerarrangement is such that the stitches of a ribbon can be placed inbetween, the ribbon eliminates the need of this electrical isolationmentioned above.

[0063] Using a ribbon in higher frequency applications also providesadvantages of the limited skin effect of ribbons compared to roundwires. For example, the skin depth at 1 MHz is 3.1 mil for aluminum and2.5 mil for copper. Since most of the current flows in a layerunderneath the conductor's surface of thickness equal to the skin depth,the reduced cross section results in higher voltage loss and higherJoule heating. For a numerical example, in a 20 mil wire, only the outerring of 3.1 mil thickness carries a significant amount of current. Itscross section is${\left( \frac{\pi}{4} \right) \cdot \left( {20^{2} - 16.9^{2}} \right)} = {90\quad {mil}^{2}}$

[0064] or approx. 90/314˜29% of the total wire cross section. Incontrast, for an 80 mil×8 mil ribbon, the cross section of the outerring of 3.1 mil thickness (neglecting the short sides) is 2×80 mil×3.1mil=496 mil² or 496/640˜77.5% of the total ribbon cross section.

[0065] Although ribbons have been used in other applications, utilizingribbons in connection with semiconductor dies and packages, such as highpower applications and MOSFETs, has not been used for numerous reasons.For example, high frequency applications, such as microwave andopto-electronic, use ribbons for its improved high frequency capability.The rectangular cross section reduces the skin effect, low loops ofappropriate shape result in low inductance of the interconnect, and theshape is more similar to the one of strip lines, resulting in lowerreflection losses at the ends of the interconnect. High frequencyapplications desire ribbons that have large surface area (reducing skineffects) and loops with well defined geometry (small variance ininductance). This leads to single ribbons with single bonds, sincespreading resistance is not an issue. Furthermore, ribbons used in thesehigh frequency applications typically use gold as the ribbon materialand processes the bonds with heating the substrate, i.e., it is athermosonic process and not a pure ultrasonic process (i.e., withoutapplication of heat).

[0066] The use of ribbon bonding of the present invention yieldsnumerous advantages. The extent of the productivity/throughputimprovement will depend on the application. For example, in a mediumpower package (e.g., a TO-220 package), three parallel 20 mil aluminumwires with two stitches on the die each were replaced with one 80 mil×10mil aluminum ribbon with three stitches on the die, for equal electricalperformance. The productivity improvement increased by a factor ofapproximately 2.5, as the process time for such a ribbon with the designcriteria (with regards to the size of the bond area) described iscomparable to that of a single 20 mil wire. In a low power package(e.g., an SO-12 package), four 5 mil aluminum wires with a single stitchon the die were replaced with one 30 mil×3 mil aluminum ribbon with asingle stitch on the die. The productivity improvement factor isapproximately 4, as the process time for such a ribbon is comparable tothat of a single 5 mil wire.

[0067] The present invention provides other features that may bebeneficial. For example, ribbon bonding yields a higher stiffness in thesubstrate plane, thereby lowering sensitivity to vibrations in thatdirection. This may have advantages in automotive applications, wheresilicon gel used to fill the power modules exerts a significant force onwires under vibration. Other advantages may result from replacingmultiple wires with a single ribbon or lower number of ribbons. Forexample, it is common to pull-test wires after bonding to determine thequality of the bond. As the number of wires on a die increases, the timeto pull-test all the wires on the die increases and/or the number ofpull-test devices needed increases. Consequently, if a single ribbon orlower number of ribbons are used instead of wires, time and/or costs maydecrease. Also, by reducing the number of total bonds or stitches on thedie, a lower yield loss potential is possible due to lower chances offorming a faulty or damaging bond on the die. It was also observed thatdue to the lower pressure, an aluminum ribbon does not penetrate as deepinto the metalized area of a die, generally <1 micron, compared to around wire of comparable hardness (>1 micron, depending on diameter andhardness). This too reduces the risk of damage to the underlyingcircuitry, offering the potential of a lower yield loss, and thereforeof lower cost of manufacturing.

[0068] Another improvement potential the rectangular ribbon geometryoffers compared to the round wire geometry is the ability to stackseveral ribbons on their bonds/stitches over each other, as shown inFIG. 11. Three ribbons 404 are shown stacked on stitches 500. Eachsubsequently stacked ribbon is bonded, e.g., ultrasonically, to acorresponding underlying bond or stitch 500. This allows theinterconnect cross section to be further increased, while still keepingthe height comparable that of a wire. For example, up to three 80 mil×8mil ribbons can be stacked this way in a power module application. Withthe trend to more efficient power semiconductors, die size is decreasingwhile current capability remains constant or even increases. Thisrequires larger cross section interconnects. If the width cannot beincreased and/or the aspect ratio must be kept at a specific value,stacking will still allow to increase the interconnect cross section.With the present cutting system, the last stitch 500-3 cannot bestacked, as the system needs a support underneath the tool during thecut move, and therefore there would not be sufficient space in adiscrete power package for this method. However this limitation could beovercome with an appropriate design of the cut mechanism. Note that thisaspect of the invention does not require stacking on ribbons withmultiple stitches; stacking over ribbons with a single bonded stitchalso provides advantages, as discussed above.

[0069] The above-described embodiments of the present invention aremerely meant to be illustrative and not limiting. It will thus beobvious to those skilled in the art that various changes andmodifications may be made without departing from this invention in itsbroader aspects. For example, the bonding of a semiconductor die to apackage is described above. However, ribbon bonding can also be betweentwo elements in an electronic module, of which one, both, or none are asemiconductor die. Further, the bonding is described primarily withregards to ultrasonic bonding, and to a lesser degree, thermosonicbonding. However, other boding processes that are suitable for use withthe flexible ribbon discussed herein may also be used, such asthermocompression. Bonding may be the of the same or different types forboth die-to-ribbon and lead-to-ribbon. Therefore, the appended claimsencompass all such changes and modifications as fall within the truespirit and scope of this invention.

What is claimed is:
 1. An electronic package, comprising: an electronicdevice having a conductive upper surface; a conductive terminal externalto the electronic device; and a conductive ribbon ultrasonically bondedto a first portion of the conductive upper surface and bonded to theconductive terminal.
 2. The electronic package of claim 1, wherein theelectronic device is a semiconductor device.
 3. The electronic packageof claim 2, wherein the semiconductor device is a power MOSFET die, adiode, or an insulated gate bipolar transistor.
 4. The electronicpackage of claim 1, wherein the conductive ribbon is bonded at multiplecontact areas of the conductive upper surface.
 5. The electronic packageof claim 1, further comprising: a second conductive terminal external tothe electronic device; and a second conductive ribbon parallel to thefirst conductive ribbon and ultrasonically bonded to the conductiveupper surface and the second conductive terminal.
 6. The electronicpackage of claim 1, wherein the conductive ribbon has a rectangularcross section.
 7. The electronic package of claim 1, wherein theconductive ribbon and the conductive upper surface are aluminum.
 8. Theelectronic package of claim 1, wherein the conductive ribbon comprises afirst layer contacting the conductive upper surface and a second layeroverlying the first layer.
 9. The electronic package of claim 8, whereinthe first layer is aluminum and the second layer is copper.
 10. Theelectronic package of claim 9, wherein the first layer is thinner thanthe second layer.
 11. The electronic package of claim 8, wherein theconductive layer further comprises a third layer overlying the secondlayer.
 12. The electronic package of claim 8, wherein the first layerand the conductive upper surface comprise the same material.
 13. Theelectronic package of claim 1, wherein the ribbon is bonded and cutalong a line parallel to a side of the conductive upper surface and theconductive terminal.
 14. The electronic package of claim 1, wherein thewidth of the ribbon is between approximately 20 mil and 100 mil.
 15. Theelectronic package of claim 1, wherein the thickness of the ribbon isbetween approximately 2 mil and 10 mil.
 16. The electronic package ofclaim 1, wherein the aspect ratio of the ribbon is approximately
 10. 17.The electronic package of claim 1, wherein the conductive terminal ispart of a semiconductor die.
 18. The electronic package of claim 1,wherein the conductive terminal is part of a lead frame.
 19. Theelectronic package of claim 4, wherein the multiple contact areas arebetween gate fingers.
 20. The electronic package of claim 1, furthercomprising a second conductive ribbon overlying the conductive ribbon.21. The electronic package of claim 20, wherein the second ribbon isultrasonically bonded to the first portion of the conductive surface.22. A semiconductor die package, comprising: a semiconductor die havingan upper conductive surface; a conductive lead external to the die; anda flexible ribbon electrically connecting the upper conductive surfaceto the lead, wherein the ribbon is bonded to the upper conductivesurface at multiple areas and wherein the ribbon forms at least one loopover the die.
 23. The die package of claim 22, wherein the die is apower MOSFET.
 24. The die package of claim 22, wherein the bonds areformed by ultrasonic bonding.
 25. The die package of claim 22, whereinthe ribbon has a rectangular cross section.
 26. The die package of claim22, further comprising: a second conductive lead external to the die;and a second flexible ribbon parallel to the first flexible electricallyconnecting the upper conductive surface to the second lead, wherein thesecond ribbon is bonded to the upper conductive surface at multipleareas and wherein the second ribbon forms at least one loop over thedie.
 27. The die package of claim 26, wherein the first and secondconductive leads are the same.
 28. The die package of claim 22, whereinthe ribbon comprises a bonding layer and a conductive layer overlyingthe bonding layer.
 29. The die package of claim 22, further comprising asecond conductive ribbon overlying the flexible ribbon.
 30. The diepackage of claim 29, wherein the second conductive ribbon is bonded toat least one of the multiple areas.
 31. The die package of claim 22,wherein the bonds are formed by thermosonic bonding orthermocompression.
 32. A method of electrically connecting an electronicdevice to an external lead, comprising: ultrasonically bonding a firstportion of a ribbon to a first portion on a conductive upper surface ofthe device; and bonding a second portion of the ribbon to the externallead.
 33. The method of claim 32, wherein the electronic device is asemiconductor die.
 34. The method of claim 32, further comprisingultrasonically bonding a third portion of the ribbon to a second portionon the conductive upper surface of the device.
 35. The method of claim32, further comprising: ultrasonically bonding a first portion of asecond ribbon to a second portion on the conductive upper surface of thedevice; and bonding a second portion of the second ribbon to theexternal lead.
 36. The method of claim 32, wherein the ultrasonicallybonding is performed with a bond tool having a diamond-shaped pattern.37. The method of claim 32, further comprising cutting the ribbon afterthe bonding.
 38. The method of claim 37, wherein the ultrasonic bondingand cutting are along parallel lines and may be performed at differentangles.
 39. The method of claim 32, wherein the ribbon comprises abonding layer and a conductive layer overlying the bonding layer. 40.The method of claim 39, wherein the ribbon further comprises a couplinglayer overlying the conductive layer.
 41. The method of claim 32,wherein the conductive upper surface is aluminum.
 42. The method ofclaim 39, wherein the bonding layer comprises aluminum and theconductive layer comprises copper.
 43. The method of claim 35, whereinthe first and second ribbon are parallel to each other.
 44. The methodof claim 34, wherein the first and second portions on the conductivesurface are between gate fingers.
 45. The method of claim 32, furthercomprising ultrasonically bonding a first portion of a second ribbon tothe first portion on the conductive surface, wherein the second ribbonoverlies the ribbon.
 46. A method of electrically connecting anelectronic device to an external lead, comprising: bonding a firstportion of a flexible conductive ribbon to a first portion on aconductive upper surface of the device; and bonding a second portion ofthe ribbon to the external lead.
 47. The method of claim 46, wherein thebonding of the ribbon is by thermosonic bonding or thermocompression.48. The method of claim 46, further comprising bonding a third portionof the ribbon to a second portion on the conductive upper surface of thedevice.